Multiplication and Motility of Salmonella enterica Serovars Enteritidis, Infantis, and Montevideo in In Vitro Contamination Models of Eggs

Abstract

The invasive ability of Salmonella enterica serovars Enteritidis, Infantis, and Montevideo in eggs was examined. Strains of these serovars originating from egg contents, laying chicken houses, and human patients were experimentally inoculated (0.1-ml dose containing 78 to 178 cells) onto the vitelline membrane of eggs collected from specific-pathogen-free chickens and incubated at 25°C. The test strains were detected in 25 of 138 yolk contents by day 6, indicating the penetration of Salmonella organisms through the vitelline membrane. There were no significant differences in overall rates of penetration between serovars. The organisms were also detected in the albumen from 125 of 138 eggs tested by day 6. Growth to more than 106 CFU/ml was observed in 48 of the 125 albumen samples. An inoculum of 1,000 Salmonella cells was added to 15 ml of albumen at the edge of a petri plate. A 10-mm-diameter cylindrical well, the bottom of which was sealed with a polycarbonate membrane with 3.0-μm pores, was filled with egg yolk and placed into the albumen at the center of the dish, which was maintained at 25°C. Experiments were performed in triplicate with each strain. Salmonella organisms in all the albumen samples were detected by day 11. However, motility of the organisms toward the yolk was observed in only two dishes inoculated with the Salmonella Enteritidis strain from a human patient and in one dish inoculated with the Salmonella Infantis strain from liquid egg. The albumen samples obtained from the dishes inoculated with the Salmonella Enteritidis strain had high numbers of bacteria (>108 CFU/ml). The present study suggests that Salmonella organisms in egg albumen are unlikely to actively move toward the yolk, although deposition on or near the vitelline membrane can be advantageous for proliferation.